1. Field of the Invention
This invention relates to a semiconductor laser device that attains laser oscillation at an extremely low threshold current level.
2. Description of the Prior Art
Conventional semiconductor laser devices are classified into two groups, gain-guided semiconductor laser devices and index guided semiconductor laser devices, according to their optical waveguiding mechanism. Index guided semiconductor laser devices are superior to gain-guided semiconductor laser devices in view of transverse mode stabilization that is important in practical use. Index guided semiconductor laser devices having a variety of structures have been proposed, in which light and current are confined within a narrow excitation region, so that the threshold current level for laser oscillation can be reduced. Typical examples of such laser devices that attain a low threshold current level are buried heterostructure (BH) laser devices and ridge-waveguide laser devices.
FIG. 2 shows a buried heterostructure laser device, which is produced as follows:
A double-heterostructure, in which an active layer 3 for laser oscillation is sandwiched between a pair of cladding layers 2 and 4 is formed on a substrate 1, and then a cap layer 5 is formed, followed by etching to form a striped mesa. The area outside of the mesa is buried with a material 14 having a low refractive index, resulting in a waveguide with a width w of as narrow as about 2 .mu.m so that a fundamental transverse mode oscillation can be achieved at a low threshold current level at about 10 mA or less. In order to effectively confine current within the mesa region that is essential to laser oscillation, the burying layer 14 must be constituted by a high resistive layer or by a multi-layer having a pn junction that is reverse to that of the mesa region.
However, since the burying layer 14 is grown after the formation of the mesa region, an energy level arises at the interface between the burying layer 14 and the mesa region, which causes ineffective current of about 2-10 mA that flows through the said interface without passing through the active region. The proportion of the ineffective current to a total amount of current increases with a decrease in the width w of the waveguide, which causes a limitation of the lowering of the threshold current level of the laster device.
FIG. 3 shows a ridge-waveguide laser device, which is produced as follows:
A double-heterostructure where an active layer 3 is sandwiched between the n- and p-cladding layers 2 and 4 is formed on a substrate 1, and then a cap layer 5 is formed, followed by etching to remove portions that reach the p-cladding layer 4 positioned above the active layer 3, resulting in a striped mesa, so that the effective refractive-index of the portion of the p-cladding layer 4 other than the mesa becomes lower than that of the mesa. The ridge-waveguide laser device is different from the BH laser device in that the entire area of the active layer 3 remains without being etched, and accordingly current injected into the ridge-waveguide laser device flows through the active layer 3. However, since a part of the p-cladding layer 4 of the ridge-waveguide laser device exists outside of the mesa, current injected into the ridge-waveguide laser device diffuses from the mesa of the p-cladding layer 4 into the portion of the p-cladding layer 4 other than the mesa, as indicated by arrow marks in FIG. 3. An amount of current, which flows into a portion other than the portion of the active layer 3 positioned below the mesa within which laser light is confined, becomes ineffective current, causing an increase in the threshold current level of the ridge-waveguide laser device.